Electrical

Question 1

Recall the power sources for the electrical system and how they are
automatically controlled:

Answer 1

Note: The integrated electrical power system (IEPS) is a fully automatic
system, requiring NO flight crew actions for normal operations.
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6.20.1 21
 Most system operation is controlled automatically by the electrical
power control unit (EPCU) and the power conversion and distribution
units (PCDU) associated with each generator.
 Power sources for the system are the integrated drive generators
(IDGs), the APU generator, external power and the batteries and
battery charger.
 The electrical system consists of a 115 volt, 400 hertz, 3 phase AC
system and 28 volt DC system
o IDGs, one for each engine and rated at 35/40 KVA continuous
supply all normal AC and DC power
o The APU generator is also rated at 35/40 KVA continuos and is
another source of AC power
 In the event of a total loss of normal AC power, three main aircraft
batteries, connected in series, will automatically power the emergency
buses and provide power adequate to maintain critical flight
instrumentation for approx. 60 minutes.

Question 2

Identify the location and function of the GROUND SERVICE ELEC PWR
switch and associated ON light:

Answer 2

OFF - Neutral position for ground service power.
 ON - Connects external power to the AC ground service bus and powers
the DC ground service bus. EXT power switch is OFF.
 ON light illuminates blue to indicate external power is powering the ground sevice bus.

Question 3

Explain the function of the EMER PWR Switch and associated ON light:

Answer 3

OFF - Emergency power is off. Resets the automatic emergency power
system.
 ARM - Emergency power automatically comes on when the EPCU detects
a power loss to the emergency AC or DC bus enabling the battery and
static inverter to power these buses.
 ON - Emergency power is on. The battery supplies power for AC
emergency bus (through the static inverter) and DC emergency bus.
 ON light illuminates amber when the emergency power system is on.

Question 4

Describe a successful EMER POWER TEST:

Answer 4

Moving the EMER POWER selector from OFF to ARM while on the ground
starts the emergency power test.
 During the test the emergency power ON light is illuminated and the EMER
POWER TEST alert is displayed on the EAD.
 The ON light extinguishes and the alert is no longer displayed at the
completion of a successful test.

Question 5

Recall the indications of a normal and non-normal battery system:

Answer 5

Green - Battery is discharging normally.
 Amber - Battery voltage is less than normal or battery discharge is
abnormal.
 Voltage digits will be boxed and amber.
 White - There is no load on the battery. Load numerics are white in all
conditions.

Question 6

Explain the difference between a No Break Power Transfer (NBPT) and a
break power transfer:

Answer 6

All electrical power source transfers normally occur without power
interruption. This is accomplished by momentarily paralleling oncoming and off-going power sources on the same bus. The Electrical
Power Control Unit (EPCU) must be functioning for a NBPT to occur.
 A break power transfer occurs when there is insufficient time to parallel
power sources. This could occur with the unexpected loss of APU or
IDG power, such as flame-out or uncommanded shutdown. Transfer of
aircraft power from the APU to an external power source may also
cause a break power transfer.

Question 7

Recall how and where generator frequency, voltage, and load values are
displayed:

Answer 7

SD Synoptic – Electrical
 Green - Generator is on with voltage and frequency within limits.
 Amber - Generator is off due to relay protective trip. Voltage, frequency,
and load current digits are boxed and amber if not within limits.
 White - Engine is not operating or generator is turned off. Voltage and
frequency digits are white if within limits or engine is not operating.
Load current digits are white if within limits.

Question 8

Recall which buses are powered when on emergency power and how
long the battery system can provide power:

Answer 8

60 minutes.
 When activated the emergency power system will provide power to the
following buses:
 Battery direct bus
 DC transfer bus
 Emergency AC bus (through emergency inverter)
 Emergency DC bus

Question 9

xplain the function of the Left and Right BUS TIE Switches:

Answer 9

Open – Respective generator cannot power the tie bus. Can also be
used to reset a bus tie lockout on the ground.
 AUTO – Generator bus to tie bus connection is controlled automatically,
to satisfy power requirements.

Question 10

Recall the function of the DC BUS TIE Switch:

Answer 10

OPEN - (Pull to unlock) Opens all three DC bus tie relays. Used during
smoke procedures or testing.
 AUTO - (Pull to unlock) The DC bus tie relays are controlled by the PCDUs
and the EPCU as required to make sure all DC buses are powered.

Question 11

Explain the function of the BATT switch and main battery system
components:

Answer 11

The main battery system consists of three-nickel cadmium batteries connected
in series. This system provides 28 volts DC power to the emergency power
system.
 OFF - (Pull to unlock) connects to the battery direct bus only. Battery
charging is not affected
.
 ON - (Rotate switch knob counter clockwise and pull to unlock)
Connects battery to the DC transfer bus and the battery direct bus.

Question 12

Recall the power source for fueling the airplane:

Answer 12

Access to the cockpit is not required to refuel and refueling can be
accomplished using battery power only.